1. Field of the Invention
This invention relates to electronic keyboard operated musical instruments and in particular is concerned with a touch responsive keyboard instrument.
2. Description of the Prior Art
Keyboard musical instruments such as the organ and harpsichord are characterized by a purely mechanical type of tone production. The musician can only control the times of tone initiation and tone cessation. The musician playing these instruments can introduce no instantaneous volume changes as an additional tone dimension to provide an expressive emotion effect to the individual musical tones. Rapid loudness control of individual tones is an inherent characteristic of most orchestral instruments. In particular, the acoustic piano has evolved to its present form and acceptance primarily because of the ease whereby a wide dynamic loudness range can be given to individual notes in response to the player's skill in actuating the key-hammer combination mechanism.
A requirement for implementing a touch responsive keyboard electronic musical instrument is a transducer to convert tactile motion into an electrical control signal. The transducers can include devices which are velocity sensitive and produce an electrical signal proportional to the speed in which a keyboard switch is depressed. Alternatively the transducer can be implemented to respond to pressure used to close a keyswitch. Transducers can also be used to sense the lateral motion of pivoted keyboard switches to provide control signals to produce tonal effects such as vibrato, glide, and pitch bend.
In U.S. Pat. No. 4,121,490 a transducer is disclosed in which the operation of a keyswitch mechanism produces an air stream having a velocity proportional to the applied force on the keyswitch. A velocity sensitive device converts the air stream motion into an electrical control signal which has a peak amplitude proportional to the peak velocity of the air flow. This signal is used to determine the loudness of the corresponding musical tone for the actuated keyswitch.
An organ-type keyboard comprises 61 keyswitches and a piano-type keyboard comprises 88 keyswitches. The number of keyswitches imposes a practical design requirement to share the electrical circuits instead of using a large number of identical circuits each of which is assigned permanently to a given keyswitch.
The output signal from most touch sensitive transducers is an analog signal. For a digital tone generator as well as for some analog tone generators, it is necessary to convert the output signal from the transducer into a binary digital number. The relatively high cost of an analog-to-digital converter has led to systems in which either one, or a small number of analog-to-digital converters are assigned on demand to transducers associated with actuated keyswitches.
A time shared method of assigning a single analog-to-digital converter to a multiplicity of transducers is disclosed in the referenced U.S. Pat. No. 4,121,490. U.S. Pat. No. 4,018,125 discloses a method of time division multiplexing a number of transducer output signals to a single converter.
The assignment of the available analog-to-digital converters should be accomplished in a fashion which minimizes the time required to generate a set of digital control signals for the array of keyboard switches. Experimentally it has been determined that all the keyboard input data should be acquired in no more than 5 milliseconds after any keyswitch has been actuated. A range of about 3 to 5 milliseconds has been found to be suitable for the data acquisition cycle time for a keyboard. This implies that the entire keyboard input switch data should be acquired in no more than 5 milliseconds and that the data acquisition cycle should be repeated with a time lapse of not more than 5 milliseconds. Since there are 88 keyswitches used in a piano keyboard, there is only 0.005/88=56.8 microseconds as a maximum time to be allotted to each key if a conventional time division multiplexed key assignor system is used to acquire keyswitch data. This is also the maximum time that would be available for an analog-to-digital conversion of the transducer signal output associated with any of the actuated keyswitches.
57 microseconds is a relatively short time to allot for an analog-to-digital signal conversion. For example, consider the situation in which note C.sub.4 has been actuated just as the time division multiplex signal has reached the immediate prior time slot corresponding to B.sub.3. This leaves only a maximum of 56.8 microseconds for the transducer's output to reach its peak value before the C.sub.4 time slot is reached and the analog-to-digital conversion is started for the touch response signal corresponding to C.sub.4. This timing situation rapidly degenerates if the C.sub.4 keyswitch happens to be actuated after the B.sub.3 time slot is started but before the C.sub.4 time slot is reached.